In general, various types of boilers used for heating have been developed and used in accordance with a required floor space or installation purpose as an oil boiler, a gas boiler, and an electric boiler in accordance with supplied fuel.
Among these boilers, particularly, in the gas boiler, as a general method for combustion of gas fuel, in the case of a pre-mixed burner, the gas fuel is combusted by mixing gas and air at a mixing ratio of an optimal combustion state in advance and then supplying mixture gas (air+gas) to a flame hole surface.
Further, in the gas boiler, a turn-down ratio (TDR) is set. The turn-down ratio (TDR) represents a ‘ratio of a minimum consumed gas amount to a maximum consumed gas amount’ in a gas combustion device in which the amount of gas is variably controlled. For example, when the maximum consumed gas amount is 24,000 kcal/h and the minimum consumed gas amount is 8,000 kcal/h, the turn-down ratio (TDR) is 3:1. The turn-down ratio (TDR) is limited according to how low the minimum consumed gas amount for maintaining a stable flame can controllably be.
In the case of the gas boiler, as the turn-down ratio (TDR) increases, convenience in heating and using hot water is increased. That is, when a burner operates in a region where the turn-down ratio (TDR) is low (that is, when the minimum consumed gas amount is large), and loads of the heating and the hot water are small, the boiler is frequently turned on and off, and as a result, a deviation in controlling a temperature is increased and durability of the device deteriorates. Accordingly, a method for improving the turn-down ratio (TDR) of the burner applied to the gas boiler has been suggested.
FIG. 1 is a graph illustrating a relationship between a consumed gas amount and pressure, FIG. 2 is a schematic diagram illustrating a combustion device in the related art, and FIG. 3 is a graph illustrating a relationship between an oxygen concentration and a dew-point temperature. A problem of the combustion device in the related art will be described with reference to FIGS. 1 to 3.
In a gas-air mixing device using a pneumatic valve, gas flows into an air supply tube by differential pressure between gas pressure of a gas supply tube and air pressure of the air supply tube to become a gas-air mixture.
Basic elements that limit a turn-down ratio (TDR) of a gas burner in the gas-air mixing device using the pneumatic valve may be a relationship between a consumed gas amount Q and differential pressure ΔP as illustrated in FIG. 1, and generally, the relationship between the differential pressure and a flow rate of a fluid is as follows.Q=k√ΔP
That is, the differential pressure needs to be increased four times in order to increase the flow rate of the fluid twice. Therefore, a ratio of the differential pressure needs to be 9:1 in order to set the turn-down ratio (TDR) to 3:1 and a ratio of the differential pressure needs to be 100:1 in order to set the turn-down (TDR) to 10:1, and there is a problem in that it is impossible to infinitely increase supply pressure of gas.
Meanwhile, in the gas-air mixing device using a gas valve of current proportional control type, the flow rate of gas has a relationship that is proportional to the square root of gas supply pressure P.
When FIG. 5 is described as an example, the differential pressure ΔP represents differential pressure between air pressure Pb of an air flow path b and gas pressure Pa of a gas path a, Pa−Pb, and it is experimentally known that when a valve at an inlet side of the gas supply tube is closed, control reliability can be secured only in the case where the gas pressure Pa of the gas supply tube is minimum 5 mmH2O or more, that is, the pressure of the gas supply tube is lower than atmospheric pressure by 5 mmH2O or more.
In order to solve a problem in that it is impossible to infinitely increase the gas supply pressure, a method has been presented, which increases the turn-down ratio (TDR) of the gas burner by partitioning the burner into several regions as illustrated in FIG. 2 and opening and closing a passage of gas injected to each burner.
In the combustion device of FIG. 2, when a region of a burner 20 is divided into a first-stage region 21 and a second-stage region 22 at a ratio of 4:6, valves 31 and 32 are mounted on the respective gas passages, and a proportional control valve 33 is installed on a supply flow path of gas in order to combust gas by controlling a supply rate of gas in accordance with fire power of the burner, a proportional control region illustrated in a table below can be acquired. In this case, it is assumed that the turn-down ratio (TDR) of each burner region is 3:1. At this time, a main valve 34 is installed at a gas inlet side of the proportional control valve 33 and the main valve 34 as an on/off valve determines whether to supply gas by opening and closing operations and is generally constituted by a drive unit.
TABLE 1Maximum gasMinimum gasClassificationamountamountFirst stage only40%13%Second stage only60%20%First stage + second stage100%33%
That is, when a maximum gas amount is 100%, since a proportional control from 13% to 100% can be achieved, the turn-down ratio (TDR) is approximately 7.7:1. However, when the combustion device having such a structure is applied to a condensing boiler, there is a problem as follows.
The condensing boiler uses a method that increases efficiency of a gas boiler by condensing vapor included in exhaust gas and collecting latent heat of the condensed vapor through a heat exchanger. Accordingly, since the vapor is more easily condensed as a dew-point temperature of the exhaust gas increases, the efficiency of the boiler is improved.
However, the dew-point temperature of the exhaust gas increases as a volume ratio (%) of the vapor included in the exhaust gas increases, and the amount of excess air (refers to oxygen and nitrogen which do not participate in a combustion reaction among constituents of the exhaust gas, H2O+CO2+O2+N2) contained in the exhaust gas needs to be small in order to increase the volume ratio of the vapor.
However, when an oxygen concentration in the exhaust gas increases (that is, the amount of the excess air increases) as illustrated in FIG. 3, the dew-point temperature rapidly decreases, and as a result, the efficiency of the condensing boiler deteriorates.
Therefore, when the region of the burner 20 is divided into the first-stage region 21 and the second-stage region 22 as illustrated in FIG. 2, air is supplied by a blower 10 up to the second-stage region 22 of the burner 20 even in the case where combustion is performed only in the first-stage region 21, and as a result, the oxygen concentration in the exhaust gas becomes very high.
Further, since the temperature of the excess air increases to a temperature of discharge gas, a part of heat by fuel combustion is used to increase the temperature of the excess air, and as a result, heat loss occurs.
Therefore, when the combustion device illustrated in FIG. 2 is applied to the condensing boiler, there is a problem in that it is difficult to anticipate high efficiency in a low-output region (that is, when combustion is performed only in the first-stage region or the second-stage region).
Meanwhile, when the pneumatic gas valve is applied, the turn-down ratio is determined depending on a blowing capability of the blower. However, since most blowers are easily controlled in a region of 1,000 to 5,000 rpm, the turn-down ratio, which can be acquired by the blower, is 5:1. In order to set the turn-down ratio to 10:1 by applying the pneumatic gas valve, the blower needs to operate in the speed range of 1,000 to 10,000 rpm, but the blower is very expensive and it is difficult to find a product commercialized for use in the gas boiler.
Further, as illustrated in FIG. 4, a type is known, which adopts a separation film A configured so that one end thereof is formed by a hinge and the other end thereof is formed as a free end for branched air flow path, such that the other end thereof can pivot around a hinge as marked with a dotted line.
However, the above type is configured so that when the other end thereof falls in a free fall scheme by a self weight, and negative pressure is applied by the blower, air flows in by a pressure difference and thus, the separation film A is lifted up by the speed of the air that flows in, and there is a problem in that, when the amount of air is variable, the separation film vibrates vertically such that an operation is instable. Moreover, when dust or foreign materials are accumulated in the hinge, there is also a problem in that the operation is not smooth.